Testing tank arranged for suppression of reflected compressional waves



April 11, `1950 w. P. MASON 2,503,400

TESTING TANK ARRANGED FOR SUPPRESSION 0F REFLECTED COMPRESSIONAL WAVES Filed 001; 6, 1945 3 Sheets-Sheet 1 TRANS- RE- CEIVER /NVENTOR W R MASON *ATTORNEY April l1, 1950 w. P, MAsoN 2,503,400

TESTING TANK ARRANGED FOR, SUPPRESSION 0F REFLECTED COMPRESSIONAL NAVES Filed Oct. 6, 1943 3 Sheets-Sheet 2 FIG. 5

COMPOSI TION HA VING LOW PV LOADED SCREEN VATZORNEV April 1l, 1950 w, MASON f 2,503,400

, TESTING TANK ARRANGED EoR SUPPRESSION oF REFLECTED coMPREss-IONAL wAvEs Filed oct. e. 1943 3 sheets-sheet s FIG. 8

STEEL wenny LIN/Na SEA WATER BUT YL RUBBER .SYNTHETIC RUBBER pc Russen ATTENIM TION IN 0B PER CENT/METER PURE GUM 0 /00 200 300 400 .500 6M 700 B00 900 [000 [[00 i200 IJDP H00 [5'00 [600 [700 FREQUENCY IN KILOCYCLES l .9V gli. A ATTORNEY Patented Apr. 101, 19570 SION WAVES OF REFLECTED COMPRESSIONAL Warren P. Mason, West Orange, N. J., .assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 6, 1943, Serial No. 505,157

7 Claims. (Cl. ISI-0.5)

This invention relates to testing devices and particularly to enclosures or containers in which acoustic energy in the ultrasonic range is transmitted from point to point.

The object of the invention is to improve the construction of a measuring tank in which electromechanical transducers .are placed under test in a manner to simulate .the conditions found in an ocean of sea vwater of infinite extent. Stated differently, an object of the invention is to provide a construction wherein tests will not be rendered false and unreliable through the reection of energy and in which only the energy intended to be directed to a receiving transducer will reach such device.

Electromechanical transducers intended for submarine Working are generally designed for the ultrasonic range Vand testing facilities must be provided. It is 'impracticable to test such devices in sea water at a .location Where there will be no reections and hence a container or tank must be provided 'for this purpose. Such tanks must be completely .lined with absorbing material so that 4energy cannot be transmitted from a source and reach the vreceiving unit except over a direct route. The absorbing material or means must prevent vreflections from the sidesof the walls of the tank. Such vabsorbing material is best realized in the form of a .combination of viscous 'liquid `and layers of a .fine mesh screen. Even so the steel walls of the tank may cause reflections which will ,direct .a disturbing ,amount of energy lback to the transducer .under test. This .is .especially true Where the tank is substantially oval in shape or .has concave-shaped Walls so that concentration of energy .takes place. Even where the walls are flat .but the ,general outline is concave this effect is found to v.provide a source o'f disturbance.

.In accordance with the present invention the walls of a tank `areso constructed that the reflections are dispersedrather than concentrated or treated in such manner that the reflections sum up to practically zero.

.One ,feature of the invention is .a tank constructed with walls-of .a convex contour whereby the .reflections are scattered and dispersed.

v.Another .feature .of `the invention is -a tank Whose inside .surfaces .are .treated in .such `a manner that reflections from one portion thereof r v2 nullify the reflections from `another portion thereof. In accordance with this feature the walls are divided into a .regular pattern of small, areas differing whereby the reections from part of these small areas are caused to be out of phase with 'the r reflections from the remainder. As a specific: example the Walls are treated in a checkerboard design with small areas .of a compound of -acous- ,1 tically reflecting material. Further, in vaccordance with this feature the walls .are vtreated in 1 a checkerboard design with some areas having. a .greater impedance than the liquid which the g` Walls contain and other areas having a lesser impedance than the said liquid. According-ly, and as controlled by the reflection factor, the reflections from these differing areas will be sub. L stantially `degrees apar-t and will therefore A rnullify .ea-ch other.

Another feature of the invention is a surface at which no reflection of compressional waves of ultrasonic frequency will take place. Still.

another feature of the invention is a method of ,rendering a .submerged body substantially invisible to range finding and submarine detecting devices working at ultrasonic frequencies.

In accordance with .another feature of the invention an Aabsorbing -means constructed of a combination of viscous liquid and massive screen. ,f

As one specic example of massive screen com- 5: prises a screen loaded with regularly disposed Weights placed thereon in a checkerboard design. f Other features will appear hereinafter. The .drawings consist of three sheets having twelve figures, as follows:

Fig. l is an outline, substantially a horizontal cross-sectional view of -one form o f measuring f tank in which concentration fof reflected waves'Av is 4prevented by the contour of `the Walls;

Fig. 2 is a .similar view of a tank having hat fwalls but being generally octagonal in outline;

Fig. .3 is a similar view fof a tank having fiat' side Walls and semi-circular ends;

Fig 4 is a simi-lar View of a tank having a simple rectangular outline; l

"Fig 5 is a checkerboard design into which a j, lining for the Walls of a tank may be cut;

Fig. 6 is an alternative V design .in which round a' holes Tare cut, the dimensions of which are such in texture from each other;

that the area of material left is equal to the area of the material cut away;

Fig. 7 is an alternative design in a diamondshaped pattern;

Fig. 8 is a view similar to Figs. 1, 2, 3 and 4 showing the tank as generally constructed in which the walls are lined, an absorbing layer of fine mesh screen in oil is used, and a barrier is used to separate the oil from the water in which the testing apparatus is immersed;

Fig. 9 is a View, greatly enlarged of a fine mesh screen into which lead blocks are molded at regular intervals for providing one practical form of massive screen;

Fig. l is a cross-sectional view of the same;

Fig. l1 is a circuit diagram showing the electrical equivalent of the loaded screen and through which calculations of the size and spacing of the lead weights can be made; and

Fig. 12 is a set of graphs showing the values of attenuation with respect to frequency which various forms of rubber will give.

The best way of making a vmeasuring tank if no absorption is available is indicated in Fig. 1. Here the four pieces of sheet steel I, 2, 3 and 4 are rolled to present convex surfaces toward the inside of the tank. They may be secured at the corners in any convenient manner as by riveting or welding. The bottom of the tank should also be formed to present a convex surface toward the interior- The tank is then filled with water, which may be either fresh water or sea water and a pair of transducers 6 and 'I immersed therein, one acting as a transmitter and the other as a receiver. Where receivers are to be measured a transmitter of known capabilities is employed and where transmitters are to be measured a receiver of known capabilities is employed. With this type of construction and due to the convex surfaces of the walls the reflection of the compressional waves from the walls is scattered and therefore the errors due to reflections are minimized. However, even with the shape reflections are appreciable, so other means to prevent reilections from the side and bottom walls must be found.

Figs, 2, 3 and 4 show other possible practical constructional forms. Fig. 3 shows the preferred form, one in which the ends are semi-circular. This form, however, acts to concentrate reflected Waves at about the points where the transducersA are located. The construction shown in Fig.f2 also concentrates the reflections at the locations ofthe transducers but not to such a high degree as-the form of Fig. 3. The simple rectangular form of Fig. 4 is practical but like the form of Fig. 2 presents certain practical diiculties in construction, so that as above stated the form 0f Fig. 3 is preferred although it does present the most perfect concave concentrating surfaces.

Applicant has found that reflections may be practically nullifled by lining the inside surfaces of.. lthe vwalls of the tank with a layer of reflecting material such as ground cork lembodied in a binder of rubber or other similar plastic material. This material is cut in a checkerboard pattern so that alternate surfaces are exposed surfaces ofthe bare steel walls. If the design is made so that the surface is one-half bare steel and the other half is lined with this reflecting material, then due to the fact that the impedance of the steel is higher than that of water, while the impedance of this lining material is lower, the waves reflected from these'twodiifering surfaces are 180 degrees out of phase and the reflections are practically and substantially nullifled.

Fig. 5 shows one form in which the lining material may be cut, a plain rectangular checkerboard pattern. Fig. 6 shows a form in which circular holes are cut, these being regularly spaced and of such dimensions that the two areas of material and of bare surface will be equally divided. Fig. 7 shows a diamond-shaped design. Other forms may be used according to other considerations such as the practical facilities for cutting and alxing to the walls' may dictate.

The lining material is in thin sheets of 40 to 80 mils thickness. Since the steel of the tank walls has a high impedance with respect to the liquid in the tank and the lining has a very low impedance with respect to the liquid the reflections from these two different kinds of surface will be approximately 180 degrees apart. In general, the effect of the lining is to scatter energy more or less uniformly throughout the volume of the tank and hence will approximate the effect of the shape of the tank of Fig. 1. A submerged body covered with a lining of this nature will be substantially invisible to range finding and locating devices working in the ultrasonic range due to the substantial absence of reflection therefrom. u

As the frequency becomes greater, the effect of the lining will be toehang'e from a uniform scattering to concentrating the'energy 'in definite directions as does a diffraction grating. For

given by Y d sin 0=nx where A is the wavelength ofthe incident wave and@J is :an integer. Hence, if the center separation is 2 centimeters and the frequency is 100 kilocycles, so that the wavelength is 1.5 centi meters, the energy will be concentrated with the lrst order spectrum at an angle of sin @=1'5=.75 0=4s4of The bare wall spaces in between will also con-y centrate their energy in the same direction, but since the path length after reflection will be only' a half wavelength from the reflection point to the, wave front and since this reflection is 180 de''" grees out of phase with the reflection from the, lined portions, the result is the combination 'of' both waves to produce concentration in one 1i-,

rection. However, this concentration will only Occur at the higherfrequencies. .It will ngt occur if the separation of the centers is equal to or less than the shorter wavelengths of interest. For the example above cited where the separa-' tion centers is 2 centimeters'the'checkerboardf will diifuse the energy in all directions'H up to-a frequency of kilocycles, which be'ing the prac-'1 tical upper limit at which some tanks `are used, i

will be satisfactory. Operation at still vhigher frequencies will require the checker-board tobe made of smaller and smaller squares.

The complete construction'of a measuring tank f which the vtransmitter I0 and receiver Il "are:

placed. This element 9 is constructed of a sound transparent plastic such as pure gum rubber, any

one of the commercial ftypesy ofl rubber such'vas lowest frequency of interest and the-permissible f reflection factor.

The construction of screens in Yoil-can-beconsidered a T network having series arms where l=" thickness of the screen y S=effective opening of a screen of 1 cm.2 ,L-viscosity ofy the oil r=`effective radius of the screen holes p= density of the oil rif-:velocity of sound VinV the oil L=separation between screens w'=',21rf (f=frequency in Y cycles per second) For the absorbing medium to work adjacent to water down to about l kilocycles and in order to reduce the reflectionffactor to alow value, the

attenuation per unit length should not be too high. This'can be expressed as A: 4,11 L 1.257f

TZSpUL L il U or lA.0835 neper per centimeterat 10 kilocycles. This condition may be approximated by having absorbing walls about 5 inches in thickness. These consist of seven screens per inch in air-free castor oil using copper wool of proper thickness as spacers. The screens Yare of 100-.mesh wire of .004 inch thickness.

The inner wall 9 is` covered with a layer I3 of absorbing material cut out as shown in Figs. 5, 6 or 7 properly proportioned with respect to the upper limit of the frequencies to be used. y

The measuring tank aboveldescribed by way of example produces useful results from 10 'kilocycles up to any higher 'frequency but below 10 kilocycles the absorbing wall ceases to function well and reflections are no longer held to a low value. The reason for this is thatthe screen materialand associated liquid load do not have enoughmass to keep the screen stationary.` As a resultA the screen moves with the oil vibrationf much as a ag waves in the breeze-and as a consequence no` damping isput in by the viscous oilscreen system. Y

This defect may be overcome by using a massive screen. While itis theoretically possible to construct screens of more massive materials the better practical method is to load the screen with a number of lead weights as indicated in Fig. 9. This can be done Aby molding the kweights i4 vaifouric'l and through the screen l5 which will give a solid connection to the screen, whereby the total mass of the screen can be very materially increased.

The question arises as'to how close `the lead weights will have to be placed together before they can be considered as a constant loading superposed on the weight of the screen. If they are too far apart all the lead weight points will remain stationary but the screen in between would vibrate in the manner of a clamped diaphragm and would be only slightly eifective in making the oil pass through the screen. It is more desirable in such a case to use a stiffer screen with a smaller numberk of meshes per inch and a more viscous liquid to get the same resistance per unit area. Such a liquid can be obtained by mixing blown castor oil, which has a high viscosity (around poises) with db castor oil'in any desired proportions. In this way the viscosity of the combination can be adjusted to any desired value.

The approximate equivalent circuit of the combination is as shown in Fig. 11 where Mn is the mass of the weights, screen and oil pulled along by the screen, C1, M1 and R1 are the stiffness, effective mass and resistance of the screen considered as a clamped diaphragm, and R2 is the resistance of the screen if its is held stationary and the oil is swished through it. Hence to be effective, we have to make the resonant frequency of the screen systems between molded weights high and the compliance C1 small. The resonant frequency has to be raised up to the point that the screen system without weights will work properly. An indication of how high the frequency and hence -how close the weights have to be spaced is given by considering the screen a solid piece of metal the same thickness as the screen with an effective radius hall the distance between weighted points. It is known that the resonant frequency of a thin plate clamped on the edges is given by the formula it is the plate thickness a is the radius Y0 is Youngs modulus p is the density, and

e is the Poisson ratio where For' a steel plate Yo=2 1012, p=7.9, and r=.3. Hence for a disc 4 mils thick, with a radius of .5

- centimeter the frequency is 10,000 cycles. This isv about the frequency at which the screen alone will have enough mass. On the other hand this calculation probably gives too high a value so it is A centimeter apart and make them .5 centimeter `on a side the mass of the lead weight per square centimeter should be 1.42 grams. The mass of the screen is .07 gram and the mass of the oil `dragged along by the screen has been evaluated at about .28 gram per square centimeter giving va total weight of 1.77 grams per square centimeter.-

The reactance of the loaded screen will equal this resistance at 1600 cycles. By making the lead loading twice as thick (l centimeter) the reactance would equal the resistance at 875 cycles.V

Hence, for 1000 cycles up a uniform attenuation with frequency would be obtained. Using such ,loaded meshes spaced at a separation of 1.2 centi-v stesso@ mfi'; lai iW'rluOni-22-r C eitmef eisz' e (2 0J ilQadd.; soreen'sriwculd provide; acloss.y for '.10 decibels :landwould result in a usable measuring tank fromfa lgousfreduencnofnloo cycles.- 1; c

eiGoineri-ofthe other extreme and .considering meansiorconstructing'a'measuring tank at very, high frequencies, :say aboreilUO kilocyclcs Ato .1,500 kilocrcles -t maybe .s uicicnt tolineithe tank an. absorbingmaterial hai/inea bien attenuationigat the frequency.of,;intcrest.. Thlls, measurements of the pro.narration characteristics. ofi oertainrrubbelsrshow the values. depictedA in, theigraphs oiFig. 12.y The-synthetic rubber there notediaS-butyl rubber nasa very high..attenuation at frequencies above 400 kilocy-clesand since this aswell kas the, other rubbers depicted allhave an excelilentfimpedance match,l to Water,titappears th tatankconstruoted accordance with Fig. 8 witv thebarrier witconstructed .of thsmateria-l would providefa-tank suitablefor hign'frequencies. Should it be desired to use the tank only f or high yfrequency measurements a simple con; struction .using a tank lined with this material alone would be sufficient, the thickness of the said lining being governed by the attenuation desired. By way of example, a lining of an eighth of an inch (about 1A; of a Icentimeter) of butyl rubber will give, an attenuation of l decibels `at 1000 kilocycles, and a lining of a full centimeter will give an attenuation of decibels.

It would be noted that the material usually employed as the barrier 9 and as diaphragrns of hydrophones is that commercial grade of rubber known as pc rubber. Where the lowest attenuation possible is desired the pure gum rubber is superior to the pc rubber at frequencies above 500 kilocycles and since this has almost exactly the same transmission characteristics as water, it may be used in the high frequency apparatus.

It will be noted that by the proper selection of the various factors hereinabove described that a measuring tank suitable for use at any given frequency or in any given frequency range may be constructed. 4

What is claimed'is:

1. In a testing device, a tank for holding a medium for the transmission of waves between a source of ultrasonic compressional waves and a transducer responsive to said Waves immersed in said medium, and means for preventing concentration of waves reflected from the walls of' said tank in the vicinity of said transducer, said means including the combination of a viscous liquid and weighted fine mesh screen between the Walls thereof and the points of location 0f said devices for absorbing waves.

2. In a testing device, a tank for holding a medium for the transmission of waves between a source of ultrasonic compressional waves and a transducer responsive to said waves immersed in said medium, and means for preventing concentration of Waves reflected from the walls of said tank in the vicinity of said transducer, said means including the` combination of a viscous liquid and ne mesh screen having weights attached thereto in a regular pattern between the walls of said tank and the points of location of said devices for absorbing waves.

3.; In a testing device, a tank, a medium in said tank for the transmission of waves, a source of ultrasonic compressional waves and a transducer responsive to said waves immersed in said rnedium with a clear and direct path through said medium between said source and said transducer and means in said tank for preventing concentrationfingthlayicinjty `ensaid" transdrceuff Wvspi rcflectedwfrom themallsnoffnsaidf. tankarsaiddastf meansincluding wall 'surfaces shaped. fto -reiiectz waves fromisaid 'Qurce mainly; away, from -said Y andi-wave fattenuatin'g devicescn-:theofN ll wavesreilected vfrom the Walls ofisaidr V .La i. i1'. .'i .f5 i.. ...,r

I agi-testing device, atank, a medium in said tankfor, thev transmission o fl Waves, a- -sourceuof ultrasonic compressional waves and a transducer' responsive to said waves immersed insaid medium: withaclear and directpath through said medumi betweengsaid source and said transducer Iand means `in said tank for preventing concentration f inirthe vicinityipr said?. transducer of `waves 1re-,.5 flected from the walls of said tank,v said -lastf means includingwall surfaces shaped to. reflect waves frovmlsaidj source mainly awayfrom` said transducer and wave attenuating devices in the paths /of all waves reflected from the walls of said tank, said attenuating devices including a plurality of layers ofone mesh screen. immersed in said medium. y

5. In a testing "'device, a tank, a medium in said tank for the transmission of waves, a source of ultrasonic compressional waves and a trans-v' ducer responsive to said waves immersed in said. medium withaI clear and 'direct path through said medium -between said sourceand said transducer and means in said tank for ypreventing conceni tration in the vicinity of said transducer of waves reflected from the walls Vof said tank, said last means including wall: surfaces shaped to reflect Waves from said source mainly away from said transducer and wave attenuating devices in the paths of al1 waves reflected from the walls of said tank, said attenuating devices including a plurality of layers of massive fine mesh screen immersedyin said medium.

61 In a testing device, a tank, a medium in said tank for the transmission of waves, a source of ultrasonic compressional waves and a transducer responsive to said Waves immersed in said medium with a clear and ldirect path through said medium between said source and said transducer and' me a`ns insaid tank for preventing ooncentration in the vicinity of said transducer of Waves reflected from the walls of said tank, said lastA means including wall surfaces shaped to reflect Wavesfrom said source mainly away from said transducer and wave attenuating devices in thel paths of all waves reflected from the walls ofr4 said'tank,"said attenuating devices including a wall surface covering of absorbing material afiixed to said walls in a regular pattern of covered and uncovered area.

7 In a testing device, a tank, a medium in said tarik for the transmission of Waves, a source ofv ultrasonic compressional waves and a transducer responsive to said waves immersed in said medium with a clear and direct path through said me v diumbetween said source and said transducer and means in said tank for preventing concen- ,i tration in the vicinity of said transducer of Waves reflected from the walls of said tank, said last means ,including wall surfaces shaped to reflect l' waves from said source mainly away from said transducer and wave attenuating devices in the paths of all waves reected from the walls of saidA tank, said attenuating devices including a wall surface covering of absorbing material afl xed to said walls in a regular pattern of covered jv and uncovered areas, said covered areas having animpedance less than the impedance of said WARREN P. MASON.

REFERENCES CITED The following references are of record in the Ille of this patent: l

UNITED STATES PATENTS Number Name Date Y Mr(Iries,l11av.`ber July 1,5, 1919 Fessenden Ot. 12, 1920 f Number 1,481,923 1,529,520

10 1 Nimm Date Nash Jan. 29, 1924 Watkins Mar. 10, 1925 Hort July 12, 1927 Paradise Oct. 9, 1928 Voigt; Dec. 29, 1931 White May '7, 1935 Pancotti Jan. 11, 1938 Percival Nov. 25, 1941 Niedermeler Aug. 25, 1942 Wood Mar. 9, 1943 Phelps ..-11;---1 May 4, 1943 

